1. Field of the Invention
The present invention relates to semiconductor pressure sensor to measure pressures, e.g., air pressure.
2. Description of the Related Art
Previously, diaphragm type semiconductor pressure sensors have been known as semiconductor pressure sensors to measure automobile tire pressures and the like (refer to Japanese Unexamined Patent Application Publication Nos. 2001-358345, 2001-352078, 2002-208708, 5-063211, 2002-071493, and 2002-350259).
FIG. 6 shows a cross-sectional structure of a semiconductor pressure sensor in a related art. This semiconductor pressure sensor includes a semiconductor substrate 110 formed from a silicon on insulator (SOI) substrate and configured in such a way that a first silicon substrate 111 and a second silicon substrate 113 are laminated with an oxide film 112 therebetween and piezo-resistance elements 103 and circuit elements 104 constitute a bridge circuit on the first silicon substrate 111. Regarding this semiconductor substrate 110, a laminated film composed of a photoresist and the like is formed on the surface (lower surface in the drawing) of the second silicon substrate 113, and the second silicon substrate 113 is dry-etched while the resulting laminated film is used as a mask, so as to form a concave portion (cavity) 122. Thereafter, the laminated film is removed through wet etching, and the semiconductor substrate 110 is bonded to a silicon substrate or a glass substrate in such a way that the concave portion 122 comes into a vacuum state.
In this regard, a diaphragm 121 is formed into the shape of a rectangle in a plan view in such a way that a piezo-resistance element 103 lies on a center of each side of the diaphragm 121. The midpoint voltage of the bridge circuit is output as a voltage based on the pressure measurement.
However, in a silicon oxide film removal step of the semiconductor pressure sensor in the related art, a silicon oxide film serving as a lower layer portion of the laminated film is removed through wet etching by using a hydrofluoric acid based etching solution or the like. At that time, the boundary portion of the oxide film 112 to the second silicon substrate 113 is over etched, so that an eroded portion 112a is formed. The size and the position of the diaphragm 121 is changed because of this eroded portion 112a. It is difficult to control the amount, the depth, and the like of the eroded portion 112a due to over etching and, therefore, it is difficult to control the size and the position of the diaphragm 121. Consequently, positional deviation occurs between the outline of the diaphragm 121 and the piezo-resistance elements 103 relatively and, in addition, an error in the shape of the diaphragm 121 occurs, so that the interrelation between the pressure and the midpoint voltage is changed and a measurement error occurs.
FIGS. 7A and 7B show the positional relationship between the piezo-resistance element 103 and the outline (each side) of the diaphragm 121. FIG. 7A shows the state of no deviation, and the piezo-resistance element is located in such a way as to lie on a central portion of each side of the diaphragm 121. FIG. 7B shows the state of the diaphragm 121 being deviated positionally (in the drawing, the state of being deviated upward and rightward). In the drawing, one side of the diaphragm 121 is specified to be 700 μm, and the dimension of the piezo-resistance element 103 is specified to be 50 μm×40 μm.
FIGS. 8A and 8B are graphs showing the results of simulation of the relationship between the pressure change (from 50 kPa to 100 kPa) and the midpoint potential in the cases where the diaphragm 121 is not deviated positionally and is deviated positionally. In FIG. 8A, the vertical axis indicates the midpoint potential (mV), the horizontal axis indicates the pressure (kPa), and the relationship between the pressure and the midpoint potential is shown where a rhombus indicates a design value, and a quadrangle, a triangle, and X indicate the relationship between the pressure and the midpoint potential in the case where the diaphragm is deviated vertically and horizontally by the amount of deviation of 5 μm, 10 μm, and 15 μm, respectively. FIG. 8B is a graph showing the result of simulation of the rate of change in full scale where the above-described diaphragm is deviated positionally. Regarding the rate of change in full scale, the ratio of the value of full scale of midpoint potential at each amount of deviation is calculated with reference to the value of full scale where the diaphragm is not deviated vertically and horizontally.
As is clear from the results of the simulation, the semiconductor pressure sensor in the related art exhibits deviation of the positional relationship between the diaphragm 121 and the piezo-resistance elements 103, and there is a problem in that the interrelation between the change in pressure and the midpoint potential of the piezo-resistance elements 103 is deviated from the design value so as to cause an error. In this regard, deviation of the size of the diaphragm 121 from the design value causes an error likewise.